L-Pyroaspartic acid ((2S)-4-oxoazetidine-2-carboxylic acid) is a head to tail cyclisation of L-aspartic acid to form the only amino acid derived azetidine. Synthetically, L-pyroaspartic acid is prepared by cyclisation of the corresponding protected esters. Unlike L-pyroglutamic acid, L-pyroaspartic acid is not a thermal degradation product but is re-named to better reflect the compound’s structural relationship to the proteogenic L-aspartic acid. Despite the intensive interest in beta-lactams spanning 70 years, L-pyroaspartic acid has received scant attention and has played more a role as a synthon to more complex molecules.